Wastewater is generally collected by gravity for treatment where the topography of the terrain will allow. In many wastewater treatment systems, a low point collection tank or wet well is used to collect wastewater for pumping from one point to the next. When water is pumped over long distances, such as over about 100 yards, biomass in the wastewater may deplete dissolved oxygen available for biological metabolism. In the absence of dissolved oxygen which is typically supplied from air, biomass may begin to reduce alternate sources of oxygen. Nitrate salts and sulfate salts, for example, are alternate sources of oxygen for biochemical metabolism.
Odor control in wastewater treatment processes is typically focused on the prevention and elimination of hydrogen sulfide. Hydrogen sulfide is a colorless and odorous gas created in wastewater collection systems by the biochemical reduction of sulfate. Sulfate is a naturally occurring substance present in most well water supplies, dissolved naturally therein through geologic contact. Sulfate reducing bacteria are commonly present in wastewater collection and treatment systems. They convert sulfate to sulfide ions in wastewater systems deficient of dissolved oxygen, a condition typically occurring when wastewater is pumped long distances. Sulfide ions in turn react with excess hydrogen ions to form hydrogen sulfide gas which has an offensive odor. Humans can sense hydrogen sulfide at levels as low as about 1 ppb in air. Hydrogen sulfide is particularly offensive in concentrations near about 10 ppm and can cause respiratory paralysis if in excess of about 1000 ppm.
Nitrate salts, such as those of sodium and calcium, are commonly used for odor control in wastewater collection and treatment systems. Nitrate salts may generally offer an alternative source of oxygen to bacteria for biochemical metabolism, such that sulfates will not be reduced, and may also be effective in removing hydrogen sulfide. Liquid solutions of calcium nitrate and/or sodium nitrate may be injected into a wastewater collection system to remove and control hydrogen sulfide downstream. Nitrate salts can also be used to remove existing sulfide present at the point of addition of nitrate material. The nitrate is generally reduced by a denitrification process to odorless nitrogen gas, which may be safely emitted from the wastewater treatment system.
Floatation of solids and/or other undesirable constituents such as fats, oils and greases in a wastewater treatment system, particularly in collection vessels such as wet wells, may cause maintenance and/or operational issues. Floating waste may be odorous, obstruct visibility, impede sampling and/or dosing, and generally interfere with treatment equipment, including pumps, hoses and level controls. Various mechanical removal techniques conventionally address this nuisance.